Steam Boilers, Engines, Nozzles and Turbines
Spontaneous combustion is a phenomenon in which

All the fuel burns instantaneously producing high energy release
Coal bursts into flame without any external ignition source but by itself due to gradual increase in temperature as a result of heat released by combination of oxygen with coal
Fuel burns with less air
Explosion in furnace

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Steam Boilers, Engines, Nozzles and Turbines
Blow off cock in a boiler is used to

Put off fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit
Increase the temperature of saturated steam without raising its pressure
Control the flow of steam from the boiler to the main pipe and to shut off the steam completely when required
Empty the boiler when required and to discharge the mud, scale or sediments which are accumulated at the bottom of the boiler

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Steam Boilers, Engines, Nozzles and Turbines
Stoichiometric quantity of air is the

Air present in atmosphere at NTP conditions
Air required to convert CO into CO₂
Air required for optimum combustion so as to have reasonable excess air
Air required for complete combustion of fuel with no excess air

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